1. Field of the Invention
The present invention relates to an information recording and/or reproducing apparatus with high density and large capacity using the principle of scanning tunnel microscope.
2. Related Background Art
Recently, memory devices and the memory systems have found diverse use in computers and associated instruments, for example the video disk and the digital audio disk, and are becoming the core of the electronics industry.
Magnetic memories and semiconductor memories have been the majority of conventional memory devices, but optical memory devices using an inexpensive high density record medium are now being developed with recent progress of laser technology. It is desired to realize a memory device or recording/reproducing apparatus having a larger memory capacity even in smaller volume in the prospect of computer use at home and information industrialization mainly of image.
In the meantime, there has recently been developed a scanning tunnel microscope (as will be referred to as STM) which permits direct observation of the electronic structure in the surface atom of a conductor. Using the STM, measurement can be done at a high resolution in real space irrespective of whether a sample is a single crystal or an amorphous material. The STM has an advantage that observation may be performed in low power and without any damage on a material to be measured, because it uses the method of detecting a micro current. Further, the STM can be operated in air or in solution as well as in super high vacuum, so that it is available for measurement of various materials and expected to be applied in various fields.
An example of applications is a study on a recording apparatus for writing information at a high resolution in a sample and a reproducing apparatus for reading information written in the sample at a high resolution, for example as described in Japanese Laid-open Patent Application Nos. 63-161552 and 63-161553.
The apparatus uses the same probe as the STM, and performs recording by applying a pulse voltage between the probe and a record medium to locally change the conductivity. The record medium employed may be a material that exhibits switching characteristics with memory function with respect to volt-ampere characteristic, for example thin film layers of chalcogenides and K-electron organic compounds. The reproduction may be conducted by detecting a change of tunnel resistance between a region thus recorded and the non-recorded region. The record medium for this recording method could be one which changes its surface shape upon application of voltage on the probe, similarly effecting recording or reproduction of information.
The apparatus employing such STM technology performs the observation while the probe electrode and the record medium are brought close to each other up to about 1 nm. Therefore, the distance must be controlled in the order of angstrom between the probe and the record medium. Further, in recording or reproducing information pieces arranged in two-dimensional matrix, two-dimensional scan of probes must be controlled in the order of several ten angstroms.
There is a proposal to simultaneously drive numerous probes (in multiple probe arrangement), enhancing a functional improvement of recording or reproducing, especially with respect to high speed processing. In such an arrangement a relative position between each probe and the record medium must be three-dimensionally controlled at the above-stated accuracies in an area in which the numerous probes are arranged. This control is conventionally effected by using a laminated piezo-electric device or a cylindrical piezo-electric device provided on the probe side or on the record medium side. These devices can ensure a large displacement amount, but are not suitable for integrated arrangement. Thus, the devices are not readily used in the recording/reproducing apparatus of multiple probe type. A solution to such a problem is disclosed in Japanese Laid-open Patent Application No. 62-281138, in which each probe is mounted on a cantilever (one-side-supported beam) with length of several hundred .mu.m and the cantilever is driven by a piezo-electric force or by an electrostatic force.
However, the information recording and/or reproducing apparatus of an STM structure with a plurality of probe electrodes needs to control the distance between each probe electrode and the record medium precisely in the order of angstrom, and has a big problem of thermal drift due to thermal expansion in the arrangement requiring a face aligning mechanism between the surface of the record medium and the surface including the tips of the plural probe electrodes.
The thermal drift would be a big hindrance in fabrication or use of a high density and large capacity recording and/or reproducing apparatus. Supposing a plurality of probes are disposed on a plane of 1 cm square and if there is a temperature difference of 1.degree. C. between the probe side and the record medium, a relative position would change by about 0.1 .mu.m on a two-dimensional plane between the probes and the record medium. In application as the recording and/or reproducing apparatus, such a positional change causes a tracking error, a reading error, or the like, resulting in a fatal defect. It is considered that a precise temperature control may be carried out using a Peltier element or the like to prevent the thermal drift. This arrangement, however, makes the thus produced apparatus complicated and expensive.